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From: kitty!larry@uunet.uu.net (Larry Lippman)
Newsgroups: comp.dcom.telecom
Subject: Loop Start and Ground Start Trunk Supervision
Message-ID: <6170@accuvax.nwu.edu>
Date: 8 Apr 90 03:26:23 GMT
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In article <5991@accuvax.nwu.edu> MAP@lcs.mit.edu (Michael A. Patton) writes:

> Loop Start provides no way to interlock against glare,
> Ground Start interlocks intrinsically in the protocol for picking up
> the trunk.

	However, glare cannot be totally eliminated, even with ground
start trunks.  A trunk for an outgoing call is selected by an idle
condition of open on the tip side of the line.  Depending upon the
type of PABX, there could well be a one second or more "window of
opportunity" for an incoming call to arrive on that very same trunk
before the PABX actually *seizes* the selected trunk by placing a
ground on its ring side.

	To quote Murphy's Law of Telephone Anomalies, "If it can
happen, it will happen, given enough traffic." :-)

> On the other hand Ground Start
> trunks can be configured (I don't remember if it's the default) to
> return positive supervision so accurate billing can be done.

	The only way that a ground start trunk can signal any type of
supervision is by means of T/R battery reversal.  Traditionally, T/R
battery reversal was provided on ground start PABX trunks as a
tariffed option *solely* for toll diversion purposes.

	I have personally never seen a PABX which considers a ground
start trunk T/R battery reversal as indicative of any type of answer
supervision, nor have I seen a tariff for this type of offering - by
New York Telephone, at least.  On the other hand, I don't exactly
claim intimate familiarity with all the new-fangled PABX's on the
market these days, so I suppose anything is possible.

> The lack of supervision also means that the PBX is solely at the mercy
> of the local user to know when a call is over.  If an outside caller
> calls you (or you call out), the switch will keep the circuit up until
> you hang up your phone, it can't tell when the outside party hangs up.
> This means that among other things --- if you leave your phone off
> hook rather than hanging up on an inbound call --- you are tying up a
> CO trunk.  In some cases the CO will detect this condition and drop
> the call on its side, opening the line up to glare or other problems.

	I have never seen a PABX without some type of CPC relay or
equivalent detector circuit which operates on CO trunk loop current.
Don't forget, loop current is loop current, whether the trunk is loop
start or ground start.  A momentary loop open on an outgoing call, as
created when a callED party disconnects, is detected by the PABX and
forces release of the station from the affected trunk.  The same thing
holds true for an incoming call, in which case the momentary loop open
is created when the callING party disconnects first.

> Now finally, in case you haven't figured it out, the above is the
> reason you can't forward incoming calls back out without Ground Start
> trunks.  The PBX has no one to watch for final disconnection.

	There is no reason why a PABX cannot consider a momentary loop
open on either the incoming trunk or outgoing forwarded trunk as the
basis to disconnect the forwarded call.  Even on a loop start line, a
momentary loop open WILL be provided when the incoming call
disconnects.  I won't disagree that the tip supervision on a ground
start trunk is better because it is a *sustained* rather than
momentary open, but there is no fundamental reason why loop start
trunks cannot function in this application.  Don't forget that
station-installed call diverters, such as those made by Ford
Industries, have been available for almost 20 years.  Such call
diverters *always* utilized two loop start lines.

> The reason I know all this is that I used to work as telecom manager
> (among other things) at a company that had a Loop Start only PBX which
> did allow forwarding to outside lines and I had to regularly go
> through all the trunks into the system to find the ones that were hung
> and fix them up (disconnection for about 5 mins seemed to do it).  

	I don't doubt your experience, but I suspect it is the result
of a PABX which was poorly designed with inadequate CPC control, or
the result of an incorrect installation (i.e., perhaps wrong wiring
options).


<> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
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